# Copyright 2011 The Emscripten Authors. All rights reserved.

# Emscripten is available under two separate licenses, the MIT license and the

# University of Illinois/NCSA Open Source License. Both these licenses can be

# found in the LICENSE file.

"""Utilties for manipulating WebAssembly binaries from python.

"""

import logging

from . import shared

logger = logging.getLogger('shared')

# For the Emscripten-specific WASM metadata section, follows semver, changes

# whenever metadata section changes structure.

# NB: major version 0 implies no compatibility

# NB: when changing the metadata format, we should only append new fields, not

# reorder, modify, or remove existing ones.

EMSCRIPTEN_METADATA_MAJOR, EMSCRIPTEN_METADATA_MINOR = (0, 3)

# For the JS/WASM ABI, specifies the minimum ABI version required of

# the WASM runtime implementation by the generated WASM binary. It follows

# semver and changes whenever C types change size/signedness or

# syscalls change signature. By semver, the maximum ABI version is

# implied to be less than (EMSCRIPTEN_ABI_MAJOR + 1, 0). On an ABI

# change, increment EMSCRIPTEN_ABI_MINOR if EMSCRIPTEN_ABI_MAJOR == 0

# or the ABI change is backwards compatible, otherwise increment

# EMSCRIPTEN_ABI_MAJOR and set EMSCRIPTEN_ABI_MINOR = 0.

EMSCRIPTEN_ABI_MAJOR, EMSCRIPTEN_ABI_MINOR = (0, 29)

WASM_PAGE_SIZE = 65536

def toLEB(x):

assert x >= 0, 'TODO: signed'

ret = []

while 1:

byte = x & 127

x >>= 7

more = x != 0

if more:

byte = byte | 128

ret.append(byte)

if not more:

break

return bytearray(ret)

def readLEB(buf, offset):

result = 0

shift = 0

while True:

byte = bytearray(buf[offset:offset + 1])[0]

offset += 1

result |= (byte & 0x7f) << shift

if not (byte & 0x80):

break

shift += 7

return (result, offset)

def add_emscripten_metadata(wasm_file):

mem_size = shared.Settings.INITIAL_MEMORY // WASM_PAGE_SIZE

global_base = shared.Settings.GLOBAL_BASE

logger.debug('creating wasm emscripten metadata section with mem size %d' % mem_size)

name = b'\x13emscripten_metadata' # section name, including prefixed size

contents = (

# metadata section version

toLEB(EMSCRIPTEN_METADATA_MAJOR) +

toLEB(EMSCRIPTEN_METADATA_MINOR) +

# NB: The structure of the following should only be changed

# if EMSCRIPTEN_METADATA_MAJOR is incremented

# Minimum ABI version

toLEB(EMSCRIPTEN_ABI_MAJOR) +

toLEB(EMSCRIPTEN_ABI_MINOR) +

# Wasm backend, always 1 now

toLEB(1) +

toLEB(mem_size) +

toLEB(0) +

toLEB(global_base) +

toLEB(0) +

# dynamictopPtr, always 0 now

toLEB(0) +

# tempDoublePtr, always 0 in wasm backend

toLEB(0) +

toLEB(int(shared.Settings.STANDALONE_WASM))

# NB: more data can be appended here as long as you increase

# the EMSCRIPTEN_METADATA_MINOR

)

orig = open(wasm_file, 'rb').read()

with open(wasm_file, 'wb') as f:

f.write(orig[0:8]) # copy magic number and version

# write the special section

f.write(b'\0') # user section is code 0

# need to find the size of this section

size = len(name) + len(contents)

f.write(toLEB(size))

f.write(name)

f.write(contents)

f.write(orig[8:])

def parse_dylink_section(wasm_file):

wasm = open(wasm_file, 'rb').read()

section_name = b'\06dylink' # section name, including prefixed size

# Read the existing section data

offset = 8

assert wasm[offset] == 0

offset += 1

size, offset = readLEB(wasm, offset)

section_end = offset + size

section = wasm[offset:section_end]

# section name

assert section.startswith(section_name)

offset = len(section_name)

mem_size, offset = readLEB(section, offset)

mem_align, offset = readLEB(section, offset)

table_size, offset = readLEB(section, offset)

table_align, offset = readLEB(section, offset)

return (mem_size, mem_align, table_size, table_align, section_end)

def add_dylink_section(wasm_file, needed_dynlibs):

# A wasm shared library has a special "dylink" section, see tools-conventions repo.

# This function adds this section to the beginning on the given file.

mem_size, mem_align, table_size, table_align, section_end = parse_dylink_section(wasm_file)

logger.debug('creating wasm dynamic library with mem size %d, table size %d, align %d' % (mem_size, table_size, mem_align))

section_name = b'\06dylink' # section name, including prefixed size

contents = (toLEB(mem_size) + toLEB(mem_align) +

toLEB(table_size) + toLEB(0))

# we extend "dylink" section with information about which shared libraries

# our shared library needs. This is similar to DT_NEEDED entries in ELF.

#

# In theory we could avoid doing this, since every import in wasm has

# "module" and "name" attributes, but currently emscripten almost always

# uses just "env" for "module". This way we have to embed information about

# required libraries for the dynamic linker somewhere, and "dylink" section

# seems to be the most relevant place.

#

# Binary format of the extension:

#

# needed_dynlibs_count varuint32 ; number of needed shared libraries

# needed_dynlibs_entries dynlib_entry* ; repeated dynamic library entries as described below

#

# dynlib_entry:

#

# dynlib_name_len varuint32 ; length of dynlib_name_str in bytes

# dynlib_name_str bytes ; name of a needed dynamic library: valid UTF-8 byte sequence

#

# a proposal has been filed to include the extension into "dylink" specification:

# https://github.com/WebAssembly/tool-conventions/pull/77

contents += toLEB(len(needed_dynlibs))

for dyn_needed in needed_dynlibs:

dyn_needed = bytes(shared.asbytes(dyn_needed))

contents += toLEB(len(dyn_needed))

contents += dyn_needed

orig = open(wasm_file, 'rb').read()

file_header = orig[:8]

file_remainder = orig[section_end:]

section_size = len(section_name) + len(contents)

with open(wasm_file, 'wb') as f:

# copy magic number and version

f.write(file_header)

# write the special section

f.write(b'\0') # user section is code 0

f.write(toLEB(section_size))

f.write(section_name)

f.write(contents)

# copy rest of binary

f.write(file_remainder)